Refactor the dao layer and update the readme with initial-load description

Purpose/Overview:
As long as different approaches are used for database communication among the code with using both PostgresClient and jooq library it brought complexity to extend the dao layer and work with it. So it is required to find all dao related methods which use plain PostgresClient and rewrite them to use jooq.
As well the initial load process and marc21_withholdings metadata prefix handling should be documented at readme file.
Requirements/Scope:

1. Update all dao methods with plain PostgresClient to use jooq
2. Update documentation with inital-load description

Acceptance criteria:

• documentation updated and clear for understanding
• code refactored
• unit tests updated